Microwave attenuator comprising aluminum oxide and aluminum titanate usable in a microwave tube



Oct. 13, 1970 R. L. HOLM EI'AL 3,534,286

MICROWAVE ATTENUATOR COMPRISING ALUMINUM OXIDE AND ALUMINUM TITANATE USABLE IN A MICROWAVE TUBE Filed May 16, 1967 //3 Press INVENTORS. Robe/7 L. Ho/m Robe/f F Bracken BYIZMM United States Patent U.S. Cl. 33381 13 Claims ABSTRACT OF THE DISCLOSURE A low porosity, metallizable, sintered ceramic body containing a portion of a nondissipative material, aluminum oxide, and a portion of a dissipative material, aluminum titanate, serves as a microwave attenuator in the 'high vacuum environment within a microwave tube. With such attenuator, there is a substantial reduction in outgassing relative to other more porous art microwave at tenuators. In the various described embodiments of the invention the porosity is low and varies from zero to only 7%, prolonging the performance of the microwave tube. Additionally, the rugged characteristics of the body indicate the ceramic to be useful for tools.

This invention relates to a dissipative or absorptive material, and more particularly, to a lossy or attenuative material for use in dissipating undesired microwave energy within an electron discharge device.

Heretofore, various materials have been utilized in microwave frequency devices for absorbing undesired microwave energy appearing therein, These materials are characterized as lossy, absorptive, or attenuative to microwave energy, and microwave energy incident thereupon is converted into heat and dissipated in such material.

As an example, a microwave tube that is capable of generating or amplifying microwave energy at a designed frequency generally possesses the capability of operating in improper modes of microwave energy and such may cause the tube to generate frequencies other than at the designed frequency. This sometimes causes destruction of the microwave tube. In order to suppress these undesired modes, absorptive or lossy material is placed at particular locations 'within the evacuated regions of the microwave tube so as to absorb or attenuate such undesired modes of microwave energy. Because the desired mode at a particularly frequency usually involves a spacial or standing wave distribution of the eletcric fields within the microlwave tube different from the spacial distribution of the electric fields within the undesired mode, a selectively placed portion of lossy, dissipative, attenuative, absorptive material, however it is termed, attenuates the undesired mode without causing any substantial loss to microwave energy in the desired mode of oscillation.

Such conventional practice has heretofore been accom lished with any one of a pluralit of known lossy materials, each having its own advantages and disadvantages. As examples, there is commonly available a nickel loaded sintered ceramic consisting of 50% nickel and 50% aluminum oxide and a carbon loaded ceramic consisting of aluminum oxide and carbon for use as attenuators in microwave tubes.

In constructing segments or shapes of lossy material for placement within the evacuated regions of a microwave tube, due consideration must be given for the practice or techniques necessary to mount the material therein. For instance, it is preferable to braze the attenuator material in place within the tube as opposed to clamping or merely disposing such material within a slot or groove in order to best protect the attenuator and tube against the effects of vibration. The attenuator material desirably should not flake or chip since the flakes or chips can cause arcing between other elements of the tube. Additionally, the attenuator material should be nonmagnetic and should not disturb the electric and magnetic fields within the tube other than by absorbing microwave power. Such attenuator should withstand high temperatures without shrinking. Of prime significance, the attenuator should not emit gases, or outgas, since the emission of gases spoils or destroys the high vacuum within the microwave tube, effectively ending the life of the tube.

Porosity in the body of an attenuator is particularly undesirable because the pores capture molecules of gas, such as oxygen, which are not released or removed from the material by vacuum pumping at high temperatures over reasonable periods of time. Because of the nature of the porous material, the gas molecules captured within the pores can escape from such material only by managing to find their way, so to speak, out of a maize. For this reason when a porous attenuator is installed in a high vacuum atmosphere within a microwave tube, many of the gas molecules eventually find their way through the porous maize into the evacuated regions of the tube to lower the degree of vacuum and, hence, lessen the per formance of the tube.

The two previously mentioned attenuators, the nickel loaded ceramic and the carbon loaded ceramic, do not possess all of the desirable characteristics of the most desirable attenuator, but have heretofore appeared to present the best compromise. The nickel loaded ceramic influences the magnetic field within the microwave tube and is generally considered non-metallizable; that is, a metal coating cannot be effectively bonded to this type of attenuator material. Accordingly, such attenuator cannot without undue difficulty be brazed in place and therefore must be clamped in place or fitted within a slot at the desired location within the microwave tube. Consequently, any vibrations that subject the ceramic to collision with a wall of the slot or relative to the support may cause flaking or chipping of the attenuator.

The carbon loaded ceramic is metallizable, and therefore can be brazed in place. However, because of its construction, it is and must be very porous. The carbon loaded ceramic is constructed from a porous or half-fired ceramic body which consists substantially entirely of aluminum oxide in powdered form that has been pressed into the desired shape and heated at a predetermined temperature, usually about 1200". It is then soaked in a solution of sugar so that the sugar molecules can seep into the pores and the body is subsequentially heated to breakdown the sugar into carbon, which is a highly lossy material. Because the porosity of the ceramic is essential to the manufacture of this type of attenuator, it retains the significant disadvantage of outgassing in a high vacuum environment.

The nickel loaded ceramic also presents an outgassing problem to a lesser degree. Because of the nature of the construction of the nickel loaded ceramic, it likewise must be porous to a substantial degree. The nickel loaded ceramic, moreover, afiects the magnetic fields within such microwave tubes.

Therefore it is an object of the invention to provide an improved microwave attenuator for use in an electron discharge device.

It is another object of the invention to provide a microwave energy dissipator capable of operating in a high vacuum region without significant outgassing.

It is a further object of the invention to provide a substantially nonporous sintered ceramic body that is metallizable, nonflaking, capable of withstanding high temperatures without shrinking, and with little or no ability to emit or release gas molecules, and is physically rugged.

It is a still further object of the invention to provide an improved electron discharge device capable of generating or amplifying microwave energy and containing a low porosity sintered ceramic microwave attenuator which does not emit or release gas molecules within the evacuated or high vacuum regions of the device.

The invention is characterized by a microwave attenuator having a substantially nonporous sintered ceramic mixture which includes as it major ingredients a portion of aluminum oxide, A1 commonly termed alumina, and another substantially complementary portion of one of the family of titanates, such as aluminum titanate, Al TiO with small amounts of other oxides such as silicon dioxide, SiO The attenuator is metallizable, nonfiaking, and withstands high temperatures in use without shrinking and possesses rugged physical characteristics.

The foregoing and other objects of the invention will become apparent from a reading of the following descrip tion together with the figure in the drawings, in which FIG. 1 illustrates an exemplary electron discharge device containing the microwave attenuator within its evacuated region and FIG. 2 illustrates a process for producing the attenuator.

FIG. 1 illustrates, in section, a partial view of a con ventional coaxial or circular electric mode magnetron. As is conventional, the magnetron contains a cylindrical cathode 1 for emitting electrons under the influence of an electric field between the cathode and an anode 2. The anode 2 is a cylindrical conductive member having a plurality of anode vanes 3, only two of which are illustrated, spaced about the inner wall of the cylindrical anode. The space between each of the vanes forms an anode resonator. Thus, a plurality of such anode resonators surround the cathode 1. A pair of pole pieces 4 and 5 are conventionally provided to establish a magnetic field therebetween in a direction perpendicular to the electric field established between the cathode 1 and anode 2. A resonant output cavity '6 surrounds the outer wall of the anode 2 and is formed in the space between the walls of the envelope and portions of the pole pieces. As is conventional in the coaxial magnetron, a plurality of slots 7 form passages for microwave energy between alternate ones of the anode resonators and the resonant output cavity. An output window 8 is provided for permitting the passage of microwave energy from output cavity 6. Additionally included in the magnetron envelope is a washer or ring-shaped microwave attenuator 9, which is brazed along its inner rim to the outer wall of cylindrical anode 2.

Many of the specific details in the construction of the coaxial magnetron, such as the mounting and placement of the magnets and the electrical sockets, are conventional and do not add to the understanding of the invention, and hence are not illustrated. Moreover, the theory of operation of this conventional magnetron is well known and is therefore not described in any detail except to state that the tube is designed to sustain oscillation at a frequency in which a TE mode of oscillation is maintained in resonant output cavity 6.

The ring-shaped microwave attenuator 9 is positioned along the corner or edge of resona t Q P 't cavity 6 in order to dissipate microwave energy of an undesired mode other than the TE circular electric mode. The wall configuration of the TE mode is toroidal in shape, and does not possess a significant electric field in that corner. However, other undesired modes of oscillation do have a substantial electric field at the location of microwave attenuator 9 and such field is incident thereupon. Microwave attenuator 9 therefore dissipates the energy in the undesired mode but does not substantially affect the energy within the TE mode.

The block of dissipative material or attenuator shown in the figure comprises a portion of aluminum oxide A1 0 and another of aluminum titanate Al TiO Small percentages of silicon dioxide and other impurities exist, but this is less than 4%. In the example shown, the aluminum oxide comprises substantially 77-80% thereof by weight with the aluminum titanate amounting to around 20% by weight, disregarding the aforesaid small percentages of impurities or binding material such as silicon dioxide or other oxide in amounts between 1-3%. The attenuator is a fired or sintered ceramic mixture of essentially the two ingredients. Additionally, the attenuator illustrated in the figure is nonporous. Consequently, it does not possess the undersirable property of outgassing. In addition, the attenuator is metallizable; that is, the sintered ceramic block can be coated with a metal on any surface to form an effective bond between the attenuator and the metal coating. Additionally, neither of the two ingredient-s is magnetic, and the attenuator therefore does not adversely efiect the magnetic fields found within electron discharge devices. The attenuator is nonflaking and nonchipping so that in use it does not release particles which can cause arcing between other elements of the microwave tube and is extremely strong and is able to withstand significant vibration. Additionally, the high temperatures used to bake out a microwave tube or the high temperatures found in normal service do not cause this attenuator to shrink.

While the proportional amounts of ingredients utilized in the embodiment of the figure are substantially 78- 80% aluminum oxide, and 20% aluminum titanate, other proportions of the same ingredients can be utilized to form other embodiments. For example, attenuators have been constructed utilizing about 87-90% aluminum oxide and 10% aluminum titanate, and other-s have been constructed with as little as 7% aluminum oxide and with up to 93% aluminum titanate. An attenuator having a 90% portion of aluminum oxide possesses all of the advantages of the attenuator in the figure except one. Because of the small amount of aluminum titanate, the amount of attenuation available for the given volume of attenuator is substantially smaller so that although the attenuator constructed with these percentages possesses all of the desirable physical characteristic-s, it does not possess the electrical characteristic in the optimum attenuator of the figure. On the other hand, attenuators constructed of 90 to 93% aluminum titanate and 7% aluminum oxide, because of the greater amount of aluminum titanate possesses better electrical characteristics per unit volume. It departs from the desirable physical characteristics in one manner. The porosity of such an attenuator may be as high as 7%. Consequently, such an attenuator can create some outgassing in use, but even so still represents a very substantial improvement over the prior art attenuators.

The term porosity as used herein is a numerical percentage defined as the wet weight of a given volume of the sintered mixture minus the dry weight of that same volume after being boiled in de-ionized water for approximately 30 minutes and having the outside surfaces wiped dry divided by the dry weight of the volume and multiplied by 100.

By comparison, acceptable microwave attenuators heretofore possess minimal porosities of about 20%, while the maximum porosity obtained in one embodiment of the present invention constructed as described and using a 93% proportion of aluminum titanate is on the order of only 7%, a relatively low porosity, while the aluminum titanate mixture and smaller proportions thereof result in substantially a zero percent or no porosity whatsoever.

A process leading to the discovery of the sintered ceramic uses the more commonly available commercial grade titanium dioxide, TiO a white colored substance in powdered form as an ingredient preliminary to the creation of the desired body of dissipative material. These steps are illustrated by the flow chart of FIG. 2. The desired proportion of commercial grade aluminum oxide 10, and titanium dioxide 11, in powdered form, are mixed 12, and pressed 13 into the desired shape. This pressed mixture is then placed into an oven 14 having a wet hydrogen or wet reducing atmosphere. The pressed mixture is then heated to at least 1500 in the wet hydrogen atmosphere. As a consequence, the white colored titanium dioxide appears to combine with the alumina to form aluminum titanate, a black colored substance. In addition, the body of material because of the heating or sintering at 1500 C. or above, becomes nonporous. The sintered mixture 15 is, of course, then removed from the oven and cooled so that it can be handled and by design is either of the desired shape and size or, if not, is cut with a grinding wheel to the size and shape desired. Additionally, the hydrogen supplied into the oven is a wet hydrogen; that is, the hydrogen gases are first passed through a bath or container of water prior to entering the oven, or air or pure oxygen in limited amounts is injected into the oven. Other ways of providing the proper atmosphere are apparent.

In one example, a commercial grade of powdered aluminum oxide (A1 0 was mixed with a commercial grade of powdered titanium oxide in the relative amounts of 90% and 10% by weight respectively. The aluminum oxide contained impurities such as silicon dioxide (SiO which acts as a binding material or interstitial bonding medium in an amount of about 2% of the 90%, providing as a practical matter, 2% silicon dioxide and 88% aluminum oxide.

The mixture was first pressed into a block and then fired in a wet hydrogen atmosphere for about half an hour at a temperature of 1550 C.

After firing, an analysis showed that the titanium oxide had combined with the aluminum oxide to form aluminum titanate in the mixture. Hence, the fired product appeared to be approximately 75.2% aluminum oxide, 22.8% aluminum titanate, and 2% silicon dioxide.

Alternatively, depending upon the availability of aluminum titanate powders, the correct proportion or percentages of the aluminum oxide and aluminum titanate, the latter of which is a black colored substance in powdered form, are mixed and pressed into a shape. This pressed body is then heated to a high temperature in a reducing atmosphere. The aluminum oxide, upon heating to temperatures in the range of 1200 C., retains its character as a porous sintered body. However, heating to temperatures of around 1500 C. and above causes closing of the pores and results in a nonporous sintered block. Thus, upon heating of the aforesaid mixture of ingredients, aluminum oxide and aluminum titanate, to a temperature of about 1500 C., a nonporous sintered body is obtained. It is then removed from the oven and cooled. It is noted that during heating process below 1500 C., the sintered block shrinks in size. This may, of course, require some design provision for change in dimension.

In the alternative, the attenuator may, of course, be formed in a large block and by use of a grinding wheel or other suitable means may be cut to the desired shape.

Conventional metallizing procedures may be used to coat a surface of the microwave attenuator with metal so that the attenuator is conveniently brazed into its place in the microwave tube with the conventional brazing procedures.

Although the illustrated embodiment uses aluminum titanate with the aluminum oxide, other dissipative materials having similar electrical and physical properties to those of aluminum titanate appear to be utilizable as a substitute. As examples, titanates of magnesium, calcium, strontium, barium, Zinc, and cadmium seem significant in that regard.

Thus it can be understood that the details describing the foregoing embodiment are presented for purposes of illustration and are not intended to limit the invention as defined by the breadth and scope of the appended claims.

What is claimed is:

1. The method of dissipating microwave energy comprising exposing to microwave energy a low porosity sintered ceramic mixture containing as the major essential ingredients aluminum oxide and aluminum titanate.

2. The invention as defined in claim 1, wherein said aluminum titanate comprises a percentage of said mixture by weight in the approximate range of 15% to 93%.

3. The invention as defined in claim 1, wherein said aluminum oxide comprises approximately 79% of said mixture by weight, and said aluminum titanate comprises approximately 20% of said mixture by weight.

4. An electron discharge device having an envelope containing a high vacuum region, means presenting microwave energy within said region, including undesired modes of microwave energy at predetermined locations therein, and a microwave attenuator disposed within said high vacuum region accessible to said undesired modes of microwave energy for dissipating undesired modes of microwave energy appearing therein, said microwave attenuator comprising a low porosity sintered ceramic mixture containing as the major ingredients therein aluminum oxide and aluminum titanate.

5. The invention as defined in claim 4, wherein said aluminum titanate comprises a percentage of said sintered mixture by weight in the range of approximately 15 to 93%.

6. The invention as defined in claim 4, wherein said aluminum titanate comprises approximately 20% of said sintered mixture by weight.

7. In combination: an evacuated chamber confining a very low pressure atmosphere in vacuum; a microwave attenuator disposed within said evacuated chamber for dissipating microwave energy incident thereon; and microwave energy generating means for generating microwave energy within said chamber, said microwave attenuator comprising a low porosity sintered ceramic mixture containing as its major ingredients a first portion by weight of aluminum oxide and a second portion by weight of aluminum titanate, said second portion by weight being less than said first portion by weight.

8. The invention as defined in claim 7 in which said aluminum titanate comprises 25% to 15 of said mixture by weight.

9. The method of dissipating microwave energy comprising exposing to microwave energy a sintered ceramic mixture essentially containing as the major ingredients thereof aluminum oxide and aluminum titanate.

10. The method of dissipating microwave frequency energy in a high vacuum region without substantial outgassing comprising exposing to microwave energy within said region a substantially nonporous body of a sintered ceramic mixture containing as the major essential ingredients a nondissipative aluminum oxide ceramic material, and aluminum titanate.

11. A method of dissipating microwave frequency energy comprising exposing microwave frequency energy to a fired ceramic body consisting essentially by weight of: A1 0 10%; Al TiO 1090%; and binder material 0-4%.

12. The invention as defined in claim 11 wherein the 2,533,140 12/1950 Rodriguez 106-39 X binder material is an oxide. 2,837,720 6/ 1958 Saltzman et a1.

13. The invention as defined in claim 11 wherein the 2,905,919 9/1959 Lorch et a1 338-224 binder is silicon dioxide. 3,169,211 2/1965 Drexler et a1 31539.77

References Cited 5 PAUL L. GENSLER, Primary Examiner UNITED STATES PATENTS 2,252,981 8/1941 Ridgway 10639 X 2,520,376 8/1950 Roup et a1. 25263.5 X 10639; 252-520; 31539.77

@2 5 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 353 5 Dated October 13, I970 I Robert L. Holm and Robert F. Bracken It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

' In Column 5, line 36, the word "oxide" should appear as -dioxide--; In Column 5, line 1 6, the word "oxide" should appear as -dioxide--.

Signed and sealed this 12th day of October, 1 971 (SEAL) Attest:

EDWARD M.FLETCHER,' JR. ROBERT GOTTSCHALK Attesting Officer Acting Commissioner of Patents 

